Individuals with irregular heart beats (arrhythmias) may need to undergo procedures to treat the heart by local intervention. Some individuals may have disabling symptoms as well as being at risk of death from their arrhythmia. During conventional procedures, a doctor will attempt to identify the pattern of electrical propagation through the walls of the heart. This is typically achieved by touching a catheter to the internal surface of the heart and recording the voltage (electrogram) at multiple positions. After an electrical ‘map’ has been constructed, ablation is typically performed by delivering radiofrequency energy to selected locations in order to create scar tissue. This scar tissue alters electrical propagation through the myocardium, hopefully treating the arrhythmia.
One of the most difficult aspects of these procedures is selecting the correct locations for ablation. The reason for this is that the surface of the heart is a complex three-dimensional structure which is traversed, during the procedure, with a probing catheter that can only record the electrogram at one position at a time.
A number of conventional techniques exist for constructing an electrical map of a heart, such as Isochronal Activation Mapping and Isopotential Mapping.
The technique of Isochronal Activation Mapping is as follows. In order to stimulate heart muscle to contract, an electrical signal travels through the myocardium like a wave. Points on a displayed image of an individual's heart are coloured according to the time when electrical activation occurs. Therefore, points that have ‘isochronal activation’ (that is, points which activate at the same time) will be displayed as having the same colour. There are a number of problems with this conventional technique including:                The catheter measures electrical activity within a small region of myocardium near its tip. However, often, there will be more than one activation time if different parts of myocardium in this region activate at different times or myocardial activation is abnormal in character and, using conventional Isochronal Activation Mapping, this complex electrical activation pattern may have to be represented at a single time point (the activation time) and there may therefore be a significant loss of information about the quality of the local activation pattern (in terms of amplitude, duration and degree of fractionation).        Errors occur in detecting the position of the electrode relative to the heart. This means that the 3D visualization of the collection of sample points is difficult. Visualization is currently aided by the display of an interpolated surface that links the sampled points. However, this can be misleading if there is a large distance between neighbouring samples. Conventional methods do not show all of the sampled points on the same surface.        Due to the above problems, an experienced member of staff is required to assist with data manipulation before the data may be displayed.        
In the conventional technique of Isopotential Mapping, the surface of the heart is displayed and coloured according to the electrogram voltage. Hence, areas with the same voltage (isopotential) have the same colour. The colour varies as the voltage changes with time. A conventional ‘true’ isopotential mapping system is the EnSite System™ of St Jude Medical. This system reconstructs the endocardial surface electrogram using inverse solutions from a far-field electrogram recorded from a non-contact intra-cardiac electrode. Conventional Isopotential Mapping has the benefit of not needing the activation time to be marked by a technician with electrophysiology experience. However, Isopotential mapping has a number of problems such as:                ‘Retraining’ the eye to interpret the colour-scales is difficult and requires a lot of practice.        All the data points are extrapolated, and not directly acquired, and therefore are susceptible to distance error and other artefact.        This technology cannot be directly applied to data obtained from a catheter that obtains electrograms at individual points. It is applicable when the voltage across the entire surfaces of myocardium is known.        
Thus there is a need for an improved system and method for recording, integrating and displaying this information which enables the time taken to perform these procedures to be reduced, enables the success rate of the procedures to be improved and patient safety to be increased by improved selection of ablation locations.